A promising application of light wave transmission lies in facilitating communication between integrated circuits within a system. Integrated circuits, such as memory chips and microprocessors, may communicate data by transmitting and receiving optical signals. Integrated circuits presently effect the transmission and reception of optical signals using Self-Electrooptic Effect Devices (SEEDs). See, Woodward, et al, "Operating characteristics of GaAs/AlGaAs FET-SEED Smart Pixels," 1992 IEDM Tech. Digest p. 655 (IEEE 1992), which is incorporated herein by reference. An integrated circuit employing SEED technology, contains arrays of optical receivers and optical modulators which receive and transmit data respectively. By employing SEED technology in many or all integrated circuits within a system, the problems and limitations resulting from the use of hardwired interconnections, such as wiring complexity, crosstalk, and parasitics may be reduced or eliminated. Such applications involve the inclusion of a substantial number of optical transmitters and receivers in one integrated circuit.
In integrated circuit or microchip design, however, as in all circuit design, the consumption of physical space incurs cost. The implementation of SEED technology requires a significant amount of the substrate surface area, as it encompasses the addition of large numbers of optical receiver and transmitter circuits. As a consequence, any improvement that operates to reduce the footprint, or surface area required by the SEED devices can result in significant financial savings, particularly in mass produced products.
Optical receiver circuits, such as those employed in SEED-based communication systems typically include a preamplifier stage comprising a light detecting device and an amplification device. The preamplifier stage receives light wave or optical signals and provides at its output an electrical signal that is representative of the received optical signal. A number of constraints apply to SEED optical receivers that do not apply in a typical photo-receiver. One constraint is the requirement of a small footprint. Another requirement is that the SEED optical receiver must produce output signals compatible with processing by electronic logic circuitry.
Two classes of preamplifier stages have gained favor with circuit designers, the transimpedance preamplifier stage and the high impedance preamplifier stage. The primary difference between the amplifiers is that transimpedance amplifiers are closed loop devices while high impedance amplifiers are open loop devices. Each class of preamplifiers has its advantages and disadvantages. High impedance preamplifiers, however, combine high sensitivity with a minimal component count, which helps satisfy the constraints discussed above. As a result, integrated circuits employing SEED technology may include an array of optical receivers, each including a high impedance preamplifier stage. Although the use of high impedance preamplifiers reduces the receiver footprint, still further reduction is advantageous.
It is well known that the footprint of a particular circuit may be reduced by optimizing component selection. It is also known that various circuit elements occupy different amounts of the substrate surface. In particular, resistors occupy a relatively large surface area in semiconductor integrated circuits. As a result, it is advantageous to substitute smaller elements for resistors in microchip-based high impedance optical receiver/preamplifiers. It is also known that a field effect transistor (FET) with its gate tied to its source may provide a suitable substitute for a resistor in certain circumstances. Because FETs are smaller than integrated circuit resistors, such as thin-film resistors, FETs are used in place of resistors when possible. FET-type devices, however, typically consist of at least three terminals, each of which requires a portion of the substrate surface.